This invention relates to an extraction process for removing glycol impurities from aqueous sodium isethionate solutions.
Sodium isethionate, which is manufactured by reaction of ethylene oxide with aqueous sodium bisulfite solution, is a key raw material in the manufacture of fatty acyl isethionate detergent compounds also known as Igepon A type surfactants. These compounds have achieved an important commercial status notably in the manufacture of detergent bars based on coconut acyl isethionate. One method of manufacturing sodium isethionate from ethylene oxide and sodium bisulfite is described in U.S. Pat. No. 2,810,747. A process for preparing coconut acyl isethionate is described in U.S. Pat. No. 3,320,292 (said patents incorporated herein by reference).
The manufacture of detergent bars based on coconut acyl isethionate involves a number of critical operating steps--one of the most important of which is a wet-mixing step during which the coconut acyl isethionate is blended at elevated temperature with water, stearic acid, soap and other minor ingredients. A description of the manufacture of such detergent bars is found in U.S. Pat. No. 2,894,912, incorporated herein by reference.
It is known that certain impurities in the sodium isethionate utilized for coconut acyl isethionate production can have a profound effect on the quality of the acyl isethionate produced. For example, the presence of sulfite in the sodium isethionate affects the odor quality of the detergent bars. Avoidance of such odor problems is the subject of U.S. Pat. No. 3,094,555 issued June 18, 1963 to the instant assignee (incorporated herein by reference).
It has now been found that the ethylene glycol content of the sodium isethionate is an important factor with respect to the processability of coconut acyl isethionate prepared from such sodium isethionate. In order to provide a product which is free from gritty or particulate matter, described as "sandy" by those in the art, it has been found that, among other conditions, the consistency of the batch during wet-mixing should be dough-like and plastic. The plastic mass undergoes a maximum of shearing and mixing in the sigma blade type mixers currently used in the manufacture of such detergent bars. If the batch, on the other hand, is liquid or "soupy" in appearance, shearing action and mixing is minimal or insufficient and leads to a notable increase in "sandy" bar production which has reduced consumer appeal and is generally commercial unacceptable, thereby requiring reworking--an expensive undertaking. Thus if the ethylene glycol content of the sodium isethionate solution is greater than about 1% by weight relative to the amount of sodium isethionate solute in the solution, the wet-mixing step in the process of U.S. Pat. No. 2,894,912 when utilizing the coconut acyl isethionate of U.S. Pat. No. 3,320,292 is adversely affected in that the batch tends to become "soupy" thereby leading to increased "sandy" bar production.
While some manufactures of sodium isethionate have been able to control the level of ethylene glycol content in their sodium isethionate production to an acceptably low level (e.g. about .ltoreq. 0.9% by weight based on the amount of sodium isethionate solute in solution), a number of manufacturers, because of equipment limitations and/or other reasons, frequently produce sodium isethionate solution well beyond this specification making it unsuitable for coconut acyl isethionate detergent bar production.
In the current era of recurring raw material shortages, especially for petrochemically-derived materials such as sodium isethionate, it has become imperative to be able to utilize all available sodium isethionate produced, whether within specification or not. While off-quality materials can often be blended with better quality product, some batches of product are so far off specification with respect to glycol content that the blending operation requires too long a period of time and leaves the sodium isethionate customer facing a shortage of an essential ingredient in his respective manufacturing process.
If the sodium isethionate solution, prepared as heretofore described, is first evaporated to dryness to form a substantially dry powder, the ethylene glycol impurity which remains in this powder can be removed readily be standard extraction using an appropriate solvent such as, for example, acetone. However, it is preferred for handling purposes to receive the sodium isethionate in the form of an aqueous solution containing about 50-60% by weight sodium isethionate dissolved therein. Such a solution at normal temperatures has no undissolved sodium isethionate, is generally clear and readily pumpable. Further, since a majo part of coconut acyl isethionate is now made by a direct reaction process from coconut fatty acids and sodium isethionate involving elimination of water (as described in U.S. Pat. No. 3,320,292), it is uneconomical in terms of energy consumption to predry the sodium isethionate, (which is hygroscopic in anhydrous form), in order to ship it and then process of a water forming reaction. Instead, it is preferred to concentrate the aqueous isethionate solution containing 50-60% by weight of dissolved sodium isethionate solute content to a higher solute content, e.g. about 73% , just prior to charging into the reactor for conversion into coconut acyl isethionate. This concentration step is performed at a sufficiently high temperature to maintain a homogeneous solution.
Accordingly, the development of efficient methods for eliminating or lowering the level of the ethylene glycol impurity in commercial sodium isethionate solutions has become an important objective for those concerned with the utilization of coconut acyl isethionate in detergent bars.